The process whereby bulk X—Ba—Cu—O material (where X equals a rare earth element such as Yb, Nd, Sm, Ho etc) high temperature superconductor (HTS) are manufactured has been the subject of considerable scientific development over the last ten years. Large grain bulk X—Ba—Cu—O materials have significant potential for generating large magnetic fields, in excess of those achievable with conventional permanent magnets, for a variety of engineering applications such as magnetic bearings, MRI and flywheel energy storage applications. Recent work has focused on doping bulk X—Ba—Cu—O with uranium oxide per se, with the uranium in various valancy states, in order to enhance the flux pinning, and hence current carrying properties of large grain bulk superconducting materials. These materials may be fabricated by a variety of processes including melt processes to produce large single grain composites. Techniques of growing the superconducting crystals are described in Volume 1, Section B2.4.3.3 of “The Handbook of Superconducting Materials” edited by Cardwell & Ginley, published by the Institute of Physics Publishing, UK. In the various melt processes available, precursor powders of X—Ba—Cu—O and UO2 are mixed either mechanically or by a solution based technique in the required stoichiometric ratios and compacted into the required geometry by uniaxial or hot cold isotatic pressing, for example. A small seed crystal of compatible crystallographic and chemical structure is usually placed on a surface of the powder compact (typically the upper surface) and the arrangement heated to the peritectically molten (i.e. partially molten) state. Alternatively, the seed crystal may be added to the compacted powders at an elevated temperature, either before or after peritectic decomposition. The sample and seed crystal is then cooled slowly through the peritectic solidification temperature during which process a single grain nucleates at the seed position and grows substantially outwards from the seed position. The decomposition and subsequent material growth processes produces uranium-doped X—Ba—Cu—O crystals. This material is fully superconducting and consists typically of a continuous superconducting microstructure (often referred to as the '123 phase) that contains discrete inclusions of a non-superconducting phase (often referred to as the '211 phase). The ability of these materials to generate or trap magnetic flux correlates either directly or indirectly with the size and distribution of the second phase inclusions. Doping uranium into the material has a significant effect on the refinement of existing inclusions and the generation of new second phase particle inclusions. Results have shown that uranium-doped second phase particle inclusions produced by co-melting of UO2 and X—Ba—Cu—O material have the general formula X—Ba—Cu—U—O. It has been demonstrated that the addition of uranium oxide per se has two key effects. These are:                a) the uranium acts as a refining agent or flux that produces a refined distribution of second phase inclusions with or without the presence of traditionally added platinum; the presence of uranium increases the achievable crystal growth rate; and        b) uranium forms a new secondary phase of particles having the formula X2Ba4CuUOz of sub-micron size throughout the crystal structure.        
Although the inclusion of UO2 particles produces advantageous X2Ba4CuUOz particles, it is not possible using known techniques to predetermine size, number etc of the second phase particles within the crystal and the processing temperatures and times for manufacturing the crystal need to be carefully adjusted to ensure that second phase particles are formed in sufficient numbers to be advantageous. Furthermore, using processes such as top-seed melt processing to produce the uranium-doped crystals by adding UO2, is relatively difficult and the conditions for processing need to be set within fairly limited range in order for the UO2 and X—Ba—Cu—O crystals to be able to tolerate the required temperatures and times in which to form second phase particles within the final crystal.
It would therefore be advantageous to provide a method of manufacturing a uranium-doped X—Ba—Cu—O type material or a X—Ba—Cu—O material doped with other elements in which the size and number of second phase particles can be predetermined before the crystal is formed. It would furthermore be advantageous to provide a method which allows increased tolerance of the manufacturing conditions and parameters and which reduces processing temperatures and times to lower cost and increase optimisation of the crystal growth process.
It would furthermore be advantageous to produce a uranium-doped X—Ba—Cu—O crystal or a X—Ba—Cu—O crystal doped with other elements, for which the first phase particles produce even stronger magnetic fields than in known uranium-doped crystals and in which addition of further dopants may increase overall performance of the crystal. It would also be advantageous to provide a doped X—Ba—Cu—O type material in which the addition of platinum, currently used to refine the particle size of doped X—Ba—Cu—O crystals, can be dispensed with without any detrimental effect to the properties of the crystals produced.
It is therefore an aim of preferred embodiments of the present invention to overcome or mitigate at least one problem of the prior art whether expressly disclosed herein or not.